1. Field of the Invention
The present invention relates to a generator of an absorption cooling system. More particularly, the present invention relates to the fluid flow pattern through the generator.
2. Description of Related Art
Typical absorption chillers have a refrigerant or working fluid consisting of at least a two-part solution, such as a solution of lithium bromide and water, or ammonia and water. Varying the solution's concentration by cyclical separation and absorption of the solution's two components allows the use of a pump, rather than a compressor, to circulate the solution through the chiller to create a cooling effect. A pump circulating a liquid solution generally requires less electrical energy or work input than other refrigerant cycles that use a compressor for compressing and circulating a comparable amount of gaseous refrigerant. However, for a given cooling effect, absorption chillers generally require more thermal energy input than other refrigerant systems that rely on compression and expansion of gaseous refrigerant. Thus, absorption chillers are often used where the savings in electrical energy out weighs the cost of the added thermal energy input.
An absorption chiller's generator uses much of the thermal energy input for separating the two components of the two-part solution. Often, a generator includes a heat exchanger in the form of coils, pipes or tubes through which steam is conveyed. For a lithium bromide and water solution, steam heat vaporizes the water out of solution, thereby leaving behind a liquid solution having a higher concentration of lithium bromide. Pressurized water vapor created by the generator then condenses upon entering an adjacent condenser. The condensed water, still pressurized, then passes through a flow restriction, which causes the water to expand to a much cooler, lower pressure water vapor. The relatively cool water vapor can then pass across a heat exchanger to cool a second fluid. The second fluid can then be used as needed, such as to cool a comfort zone of a building. The cool water vapor is subsequently reabsorbed by the higher concentrated liquid solution from the generator. This creates a solution of intermediate concentration, which is pumped back to the generator to complete the cycle.
In designing a generator for an absorption chiller, several factors need be considered. For example, lithium bromide and other absorption solutions can be expensive, thus it is often desirable to minimize the total amount of the solution in the chiller. One way to do this is to reduce the amount of the solution in the generator. However, insufficient solution in the generator can reduce a generator's efficiency, due to less heat transfer area. Insufficient solution can also create "hot spots" or areas where the steam coils of the generator rapidly heat a relatively small portion of solution. This can cause rapid, localized boiling of the solution, which can create a problem known as carryover.
Carryover is a generally undesirable effect where vaporous water entrains concentrated liquid lithium bromide solution from within the generator, and then carries the liquid over into the condenser. In some cases, evenly distributing the steam heat to the solution by increasing the liquid level of the solution in the generator can help reduce carryover. Of course, such an approach to solving the carryover problem unfortunately increases the total amount of solution in the generator.
Another problem often associated with steam-heated generators is a phenomenon known as water hammer. Water hammer is when the tubes or pipes that convey the steam through the generator produce a banging or hammering sound, due to superheated steam flash-heating condensate within the pipes. The resulting rapid expansion of the condensate causes a shockwave that creates the hammering noise. If the temperature of the steam is reduced in an attempt to avoid water hammer, the rate of heat transfer from the steam to the solution may be reduced as well. Yet, raising the temperature of the steam to reduce the amount of condensate within the steam pipes can reduce the efficiency of the generator.
Consequently, a need exists for a more efficient absorption generator that minimizes carryover, water hammer, and the e amount of solution in the generator.